Reference · Printable
Scale ruler
An architect’s scale rule reads real-world distances straight off a scaled drawing. Choose the ratio your drawing was made at — 1:50 and 1:100 cover most of them — print at 100%, and measure without doing any arithmetic.
Which scale for what
The third column is the practical test: how much paper one real metre occupies. Below about 2 mm per metre the drawing stops being measurable by hand and becomes a location reference instead.
| Scale | Typically used for | 1 metre on paper |
|---|---|---|
| 1:20 | Construction details, joinery, stair sections | 50 mm |
| 1:25 | Detailed room plans, kitchen and bathroom layouts | 40 mm |
| 1:50 | Floor plans, sections and elevations — the everyday architectural scale | 20 mm |
| 1:100 | Whole-building plans, apartment layouts, planning submissions | 10 mm |
| 1:200 | Large buildings, schools and hospitals on a single sheet | 5 mm |
| 1:500 | Site plans and building footprints in their plot | 2 mm |
| 1:1000 | Site location plans, small masterplans | 1 mm |
| 1:1250 | Location plans for planning applications (UK convention) | 0.8 mm |
| 1:2500 | Large site and land-registry plans | 0.4 mm |
Why the ratio is written that way
“1:50” means one unit on the drawing stands for fifty of the same unit in reality — the units cancel, which is why the notation carries no millimetres or inches and works identically for a plan printed in Athens or Auckland. It also means the ratio is only true while the drawing keeps its printed size. Rescale the sheet and the ratio silently changes with it, which is the single most common source of wrong measurements taken off a printed plan.
The same logic drives the scaling percentages in our paper size converter: moving an A3 drawing onto A4 at 71% turns a 1:50 drawing into a 1:70.7 drawing. If you must reduce a drawing, note the new ratio on the sheet — or print a matching scale ruler and staple it to the drawing.
Frequently asked questions
What does a scale ruler actually do?
It removes the arithmetic. On a 1:50 drawing, a wall measuring 20 mm on paper is one metre in reality — a scale rule has that conversion built into its markings, so you read "1 m" directly instead of measuring 20 mm and dividing. Each face of a physical scale rule carries a different ratio; here you print the one you need.
Which scale should I use for a floor plan?
1:50 for a room or apartment you want to see in detail, 1:100 for a whole house or a planning drawing that must fit one sheet. The rule of thumb is that 1:50 shows door swings and furniture comfortably, while 1:100 shows the building shape and room relationships. Below that, 1:200 and 1:500 are for the building in its site rather than the rooms inside it.
How do I read a scale ruler?
Line the zero mark up with one end of what you are measuring and read the number at the other end — that number is already in real-world metres. The small divisions between the numbered marks subdivide the metre, typically into tenths, so a reading between 3 and 4 with two small marks past 3 is 3.2 m.
Why does print accuracy matter more for a scale ruler?
Because the error multiplies. If your printer shrinks the page by 3%, a life-size ruler is visibly wrong the moment you hold a real ruler against it. A 1:100 scale ruler with the same 3% error looks perfectly normal and silently reports every distance 3% short — 30 centimetres of real building for every 10 metres measured. Print at 100% / Actual size and check against a known ruler first.
Does the drawing itself have to be printed at the right scale?
Yes, and this is the mistake that catches people out. A 1:50 drawing printed "fit to page" on A4 when it was drawn for A3 is no longer at 1:50, and no scale rule can save it — the printed drawing is at some arbitrary ratio. Check the drawing has a printed scale bar and measure it before trusting any dimension you take off the sheet.
Are imperial scales supported?
Not here. Imperial architectural scales work differently — they are expressed as ¼ in = 1 ft rather than as a ratio, and a proper imperial scale rule divides the inch into twelfths for feet and inches. Rather than approximate that badly, the scaled ruler is metric only; the life-size setting still prints inches down to sixteenths.